From what I read in postings about uv lights, you want slower flow through them. I have also read that you can run them just while the green bloom is going then turn it off. They don't need to run all of the time.
Do you have fish in there? if so be careful about stirring up the muck, it will stink and has anaerobic bacteria that can harm your fish. At least with a trash pump it will get sucked up and out. I saved some fish from a foreclosure, that water stunk had a oil slick on it when we were trying to net the fish.
The plants in the stream will help with filtering the water. I just filter with a bog full of plants, no algae issues last year, none so far this year.